The role of the nodular lymphoid tissues of the rabbit gut will be evaluated by studying antibody responses to sheep erythrocytes, B gamma G, endotoxin (LPS), and carbohydrate antigens, both in thymectomized, appendectomized, lethally irradiated animals restored with autologous and/or allogeneic thymus, appendix, or marrow cells in various combinations, and also in primary cultures of the same cell populations. The presence of immunoglobulins on appendix cells will be sought by conventional techniques and these will be characterized as to class, allotype, and specificity. Cells repopulating the appendix after irradiation and marrow transfer (or shielding) will be examined for competence as above and for the presence of immunoglobulin. The role of LPS in stimulating germinal center formation will be tested by stimulating, in vivo or in vitro, the appendix cells of similar animals in which cecal ligation and appendicostomy, with sterilization of the lumen, are carried out. The role of recirculating lymphocytes in supplying competence to the bone marrow or the appendix used in restoration experiments will be tested by prolonged thoracic duct drainage of suitable donors. Heteroantisera will be prepared against thymus and appendix cells, in a attempt to develop specific reagents for rabbit T and B-lymphocytes by the use of cytotoxicity assay and suitable absorptions.